1. Field of the Invention
The invention relates to a label film that is formed by co-extrusion and has a layer that forms a surface on a first side, the layer being composed of polyethylene with a proportion of at least 70 wt.-%, and preferably at least 85 wt.-% high-density polyethylene, and the film thickness amounts to less than 75 μm. The invention particularly relates to multi-layer co-extruded films, whereby the layer that forms the surface on the first side represents a cover layer. In the case of a multi-layer configuration, at least two cover layers that lie on the outside, with a core layer disposed between them, are usually present.
2. The Prior Art
Label films are usually provided with an adhesive on one of their sides, and are kept available as a label film laminate, for the time being, together with a backing that covers the layer of adhesive. Labels are punched out of the material of the label film, so that these can be individually applied after the backing is pulled off.
The present invention particularly relates to a label film that is suitable for processing in a usual labeling apparatus, whereby separation of the label film from the backing takes place by angling it away and subsequently pulling-off the backing. For this purpose, the label film must demonstrate sufficient bending rigidity, whereby at the same time, for cost reasons, not only a low film thickness but also the use of the most cost-advantageous raw materials possible are aimed at. Furthermore, the label film must demonstrate good punching properties and must also be imprintable. Particularly in the case of higher-quality products, the label film must also have a high-quality appearance and must show an imprint to good advantage. Aside from sufficiently great bending resistance, great tear resistance and a tear propagation resistance that is balanced out in different directions/production directions and the transverse direction are aimed at, in order to achieve the greatest possible strength and good handling of labels that are formed from the label film.
Depending on the product provided with the label or the product packaging provided with the label, a matte or shiny surface can be advantageous. In many application cases, it is desired that the label itself is not visible or only barely visible, and the product is visible in the non-imprinted region, or, if applicable, the product packaging is visible, if this packaging is not itself transparent. Also, shiny surfaces as a whole are more popular than matte surfaces.
A label film having the characteristics described above is described in European Patent No. EP 1 543 955 B1. In the case of a label film consisting entirely of polyethylene, a reduction in the total thickness can be achieved by providing a core layer composed of a low-density polyethylene having a density of less than 0.945 g/cm3 between two outer cover layers composed of a high-density polyethylene having a density of more than 0.945 g/cm3, with the outer layers having a layer thickness of at least 15 μm. The embodiment described is based on the recognition that high-density polyethylene is more rigid as compared with low-density polyethylene, and the use of a rigid material in the outer zones has a greater influence on the bending rigidity of the label film. This is because when the film is bent, the outer sides are expanded or compressed more greatly, while the deformation about a central, neutral fiber is comparatively slight. The label film known from EP 1 543 955 B1 has good mechanical properties at a low total thickness. However, in practice, the optical properties known from conventional label films, particularly great shine and low cloudiness, cannot be achieved, so that the area of use of the label film described in EP 1 543 955 B1 is accordingly limited.
A label film having a reduced thickness is known from International Application No. WO 2009/010079 A1, which film is produced by co-extrusion. In the case of an arrangement having two cover layers, the core layer disposed between them has at least 50% polypropylene, thereby achieving sufficient bending rigidity at a thickness of less than 80 μm, in order to be able to process the label film in a usual labeling apparatus. Different polyethylene types and polymer mixtures with styrene-butadiene-styrene copolymer are proposed as cover layers. The embodiment of the core layer provided according to WO 2009/010079 A1, with at least 50% polypropylene, leads to the result that the entire label film cannot be punched as well, because of the increased strength of the polypropylene. In addition, there are also significant disadvantages with regard to the extrusion process, because on the basis of the higher melting point of polypropylene, a higher processing temperature and accordingly also a greater introduction of heat are required. In addition to the greater demand for energy on the basis of the increased melting and extrusion temperature, a longer cooling time in connection with extrusion must also be taken into consideration accordingly, and this has a negative effect particularly for blown film extrusion, in which the extruded film is at first present as a type of bubble. Finally, process management and the apparatus expenditure in the case of materials having different melting temperatures, such as PE and PP, are significantly increased. Another label film having outer layers composed of polyethylene and a core layer composed of polypropylene is known from German Patent No. DE 198 59 789 C1, whereby here, too, the disadvantages described above are present.
In practice, multi-layer label films are predominantly used, which have a thickness between 80 μm and 90 μm. The individual layers are each formed from a blend of low-density polyethylene (LDPE) and high-density polyethylene (HDPE). In this connection, the main component of the individual layers is LDPE, and HDPE is usually mixed in at a proportion of less than 30 wt.-%. The percentage proportion of HDPE varies as a function of the desired optical properties, and the shine and the transparency of the label film are improved by a reduced proportion of HDPE in the cover layers, as compared with the core layer. The standard label films described have good optical properties, easy punchability, good mechanical properties, as well as good imprintability. For cost reasons, such standard label films are generally produced as blown films.